1. Field of the Invention
The invention relates to a method and a circuit for checking the width of the air gap between a coded rotor, which varies a magnetic flux, and at least one sensor element which is sensitive to magnetic fields and is provided for recording the rotary movement of the rotor, and to which a circuit is connected and in which measurement signals are generated by the magnetic variation during rotation of the rotor, the measurement signals being converted by the circuit into output signals which can be evaluated by a connected control device.
2. Description of the Related Art
Magnetically based speed sensors are used, for example, in motor vehicles in connection with antilock systems. A circuit for such a sensor is known, for example, from WO 96/10751. The circuit changes the shape or the strength of the output signal as a function of the air gap width. The output signal is evaluated by the connected control device. However, the expenditure for the evaluation is very high in the case of such a mode of procedure, since the output signal is not present in discrete form. Moreover, the direct modulation of information on direction of rotation is an unreliable method, since interference has a strong influence.
It is the object of the present invention to specify a method and a simple, cost-effective circuit for a magnetic speed sensor for generating a discrete speed signal which contains information for the width of the air gap, two lines sufficing to relay this signal to other devices via a cable connection.
This object is achieved by means of the method according to the invention by virtue of the fact that square-wave output signals are generated from the measurement signals by means of at least one prescribable threshold value, wherein the pulse duty factor of the square-wave output signals is kept within a prescribed range in the case of an air gap width lying within a tolerance band, and wherein the pulse duty factor deviates in a reliably measurable fashion from the prescribed range in the case of amplitudes of the measurement signal outside prescribed limits.